Definitions:

"HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a bidirectional serial/parallel high-bandwidth, low-latency point to point link that was introduced on April 2, 2001.[1] The HyperTransport Consortium is in charge of promoting and developing HyperTransport technology. The technology is used by AMD and Transmeta in x86 processors, PMC-Sierra, Broadcom, and Raza Microelectronics in MIPS microprocessors, AMD, NVIDIA, VIA and SiS in PC chipsets, HP, Sun Microsystems, IBM, and IWill in servers, Cray, Newisys, QLogic, and XtremeData, Inc. in high performance computing, Microsoft in its Xbox game console, and Cisco Systems in routers. Notably missing from this list is semiconductor giant Intel, which continues to use a shared bus architecture."

Applications for HyperTransport:

Front-Side Bus Replacement:
The primary use for HyperTransport is to replace the front-side bus, which is currently different for every type of machine. For instance, a Pentium cannot be plugged into a PCI bus. In order to expand the system, the front-side bus must connect through adaptors for the various standard buses, like AGP or PCI. These are typically included in the respective controller functions, namely the northbridge and southbridge. In theory, a similar computer implemented with HyperTransport is faster and more flexible. A single PCI↔HyperTransport adaptor chip will work with any HyperTransport enabled microprocessor and allow the use of PCI cards with these processors. For example, the NVIDIA nForce chipset uses HyperTran sport to connect its north and south bridges.

Multiprocessor interconnect:
Another use for HyperTransport is as an interconnect for NUMA multiprocessor computers. AMD uses HyperTransport with a proprietary cache coherency extension as part of their Direct Connect Architecture in their Opteron and Athlon 64 FX (Dual Socket Direct Connect (DSDC) Architecture) line of processors. The HORUS interconnect from Newisys extends this concept to larger clusters.

"Memory controller: a chip on a computer's motherboard or CPU die which manages the flow of data going to and from the memory.
Most computers based on an Intel processor have a memory controller implemented on their motherboard's north bridge, though some modern microprocessors, such as AMD's Athlon 64 and Opteron processors, IBM's POWER5, and Sun Microsystems UltraSPARC T1 have a memory controller on the CPU die to reduce the memory latency. While this has the potential to increase the system's performance, it locks the processor to a specific type (or types) of memory, forcing a redesign in order to support newer memory technolgies. When DDR2 SDRAM was introduced, AMD released new Athlon 64 CPUs. These new models, with a DDR2 controller, use a different physical socket (known as Socket AM2), so that they will only fit in motherboards designed for the new type of RAM. When the memory controller is not on-die, the same CPU may be installed on a new motherboard, with an updated northbridge."